The subject invention is directed toward the art of electrolytic cells, especially alkali metal chlorate or hypochlorite and diaphragm-type chlor-alkali cells and, more particularly, to an anode assembly for use in such cells and a method and apparatus for forming the same.
Cells used for electrolysis of alkali metal halide brines usually employ a perforated metallic cathode. In chlor-alkali cells, a fluid-permeable diaphragm overlies the cathode and permits hydraulic flow of electrolyte from the anode chamber through the diaphragm and cathode into the cathode chamber.
Somewhat recently, dimensionally stable anodes have been developed for use in these cells. For example, U.S. Pat. No. 3,591,483 to Richard E. Loftfield and Henry W. Laub describes several embodiments of dimensionally stable anodes and methods and apparatus for efficiently and simply mounting the anodes.
In general, the anodes described in the noted patent normally comprise an anode riser or conductor bar which supports members having an extended electrically conductive surface. The surface can comprise any of several materials which have a sufficiently low chlorine overvoltage and which are chemically inert to the electrolyte and resistant to the corrosive conditions in the cell. The many possible materials which can form the electrically conductive surface are discussed at length in the patent but, typically, comprise alloys of platinum group metals, oxides thereof, and mixtures of the metals and oxides.
The electrically conductive surface is normally supported from a sheet or member of a valve metal or alloy thereof; e.g., a film-forming metal such as titanium, tantalum, zirconium, niobium and the like. In current commercial embodiments, the electrically conductive surface is supported on perforated or foraminous sheets, expanded metal of titanium. Typically, two of the sheets are joined in parallel on opposite sides of the anode riser. The anode riser itself is normally a titanium-clad copper electrode. In an attempt to maintain parallelism and flatness of the sheets, as well as to insure structural rigidity and integrity, various types of cross braces and the like have been joined between the sheets. Generally, the components making up the anode assembly, including the braces, are joined by welding.
In order for the anode assembly to function satisfactory, it must have certain mechanical-structural characteristics in addition to the chemical and electrical characteristics discussed in the aforementioned patent. For example, the sheets supporting the electrically conductive surface must have a high degree of flatness. And, the flatness must be maintained throughout extended periods of operation. This is required because irregularities in the surface vary the spacing between the anode assembly and the associated cathode assemblies result in variations in current flow and reductions in the overall efficiency of the cell. Closely associated with the surface flatness requirement is the necessity for extreme parallelism and uniformity of spacing between the sheets of the anode assembly. Variations in these parameters have the same general effect on cell performance as lack of flatness.
In addition to the required dimensional uniformity, the joints and/or bonds between the various components must exhibit uniform electrical conductivity as well as structural strength. This has best been achieved by joining the components with a series of closely-spaced spot welds. However, the difficulties encountered in welding the assemblies have been substantial. First, titanium is inherently difficult to weld. Secondly, the presence of the electrically conductive surface of oxides greatly increases the welding difficulties. Moreover, the welding operation itself tends to result in warpage in the assembly. Consequently, the assembly must be straightened following the welding. In most instances, this has been a time-consuming manual operation.
The addition of internal braces or cross members between the sheets in an attempt to reduce the warpage problem merely produced further manufacturing problems with little or no lessening of warpage or mechanical distortion problems.